The structure of multi-axion solutions to the strong CP problem

TL;DR

This paper explores the diverse phenomenological patterns of multi-axion solutions to the strong CP problem, revealing new possibilities beyond single-axion models and guiding future experimental searches.

Contribution

It provides a general framework for understanding multi-axion solutions, including sum rules and phenomenological regimes, extending beyond traditional single-axion assumptions.

Findings

Identifies new mass-coupling patterns for multi-axion systems.

Derives a sum rule for N-axion systems incorporating PQ breaking and anomaly coefficients.

Highlights UV-complete theories that naturally produce different phenomenological regimes.

Abstract

A broad experimental program is targeting the QCD axion band predicted by single-axion solutions to the strong CP problem. Multi-axion theories provide a well-motivated departure from this canonical picture, since additional states generically modify the mass-photon-coupling relation. We investigate the general structure of multi-axion solutions to the strong CP problem and study the different qualitative mass-coupling patterns that arise, including axions to the right of the QCD band, axions in the experimentally accessible region to its left, and scenarios in which the QCD axion band itself is displaced. This general treatment reveals a broad set of phenomenological possibilities that are not captured by more restrictive assumptions. In particular, we identify the structure of Peccei-Quinn symmetry breaking and the relative alignment between the QCD and electromagnetic anomalies as key ingredients determining the location of the axions in parameter space. Combining these ingredients, we derive a general sum rule for $N$-axion systems that incorporates both general PQ breaking and non-universal anomaly coefficients. We apply the framework to the two-axion system and to general multi-axion setups, identifying UV-complete theories in which the different phenomenological regimes arise naturally. Our results motivate an extended axion search program and have implications for our understanding of fundamental physics and the ultraviolet completion of the Standard Model.